Reduce handler request boilerplate

- Add session.Attach to reuse per-request sessions across middleware and ghome handlers
- Make Session finalize/abort paths idempotent to avoid duplicate commit or rollback work
- Move request_data parsing helper to internal/utils
- Expand ParseRequestData usage across api, download, unit, live, multiunit, and subscenario handlers
- Extract download package metadata into internal/handler/download/types.go
- Return explicit errors from selected session lookup helpers and update callers
- Make api action handlers return errors directly instead of calling ss.CheckErr in nested layers

Signed-off-by: Sean Du <do4suki@gmail.com>

internal/session/deck.go

@@ -4,25 +4,28 @@ import (
 	usermodel "honoka-chan/internal/model/user"
 )
 
-func (ss *Session) GetUserDeck(deckID int) (bool, *usermodel.UserDeck) {
+func (ss *Session) GetUserDeck(deckID int) (bool, *usermodel.UserDeck, error) {
 	deckData := usermodel.UserDeck{}
 	has, err := ss.UserEng.Table("user_deck").
 		Where("user_id = ? AND deck_id = ?", ss.UserID, deckID).Get(&deckData)
-	if ss.CheckErr(err) {
-		return false, nil
+	if err != nil {
+		return false, nil, err
 	}
 
-	return has, &deckData
+	return has, &deckData, nil
 }
 
 func (ss *Session) GetUserDeckUnit(deckID int) []usermodel.UserDeckUnit {
-	has, deckData := ss.GetUserDeck(deckID)
+	has, deckData, err := ss.GetUserDeck(deckID)
+	if ss.CheckErr(err) {
+		return []usermodel.UserDeckUnit{}
+	}
 	if !has {
 		return []usermodel.UserDeckUnit{}
 	}
 
 	unitData := []usermodel.UserDeckUnit{}
-	err := ss.UserEng.Table("user_deck_unit").
+	err = ss.UserEng.Table("user_deck_unit").
 		Where("user_deck_id = ?", deckData.ID).Find(&unitData)
 	if ss.CheckErr(err) {
 		return []usermodel.UserDeckUnit{}